1. Field of the Invention
This invention relates to a data transmission system and more particularly to a system using an implicit token passing method.
2. Description of the Related Art
The LANs (local Area Networks) which have been developed in recent years are now rapidly being standardized. Typical methods that have been developed include a CSMA/CD (Carrier Sense Multiple Access with Collision Detection) method (IEEE standard 802.3), a token bus method (802.4), and a token ring method (802.5). The CSMA/CD method was the first of these to be applied to practical use, and it has a high level of reliability. However, there is a problem with the CSMA/CD method, since collisions between transmission frames from different transmission nodes sometimes occur,and therefore response is not guaranteed. In particular, the CSMA/CD method is not suitable for FA (Factory Automation) or LA (Laboratory Automation), where real-time characteristics are essential.
A proposal was made recently to overcome this defect in the CSMA/CD method by adding real-time characteristics through the use of the implicit token passing method (Japanese Patent Application No. 58176385). An explanation of this conventional implicit token passing method is given below. FIG. 1 (Prior Art) shows how a data transmission system is configured such that all the stations 20 (Nos. 1-N) are connected to a common transmission line 1. For the sake of convenience, a bus-type system is illustrated here, but there is no restriction as to the topology of the transmission network form. FIG. 2 is a time chart, provided to explain the working of this system. One of the stations (Nos. 1-N), acting as the master station, notifies the other stations that a transmission cycle is beginning by outputting a frame header (FH) 21, which is an indicator to start a transmission cycle. In the example shown in FIG. 2, No. 1 is the master station. Each station incorporates a slot counter. This slot counter is incremented every time its station receives a frame 23 on the transmission line. This slot counter is reset when a frame header (FH) 21 is received. When the value of the count in this slot counter becomes one less than the number assigned to each station, the transmission of frames (F) 23 of the preceeding station has been completed and after the elapse of a time-space 25 between the frames, each station is allowed to transmit its own data. When the last station (No. N) has finished transmitting its frames (F) 23, the master station (No. 1) moves to the next transmission cycle by transmitting another frame header (FH) 21. The adoption of this implicit token passing method provides the CSMA/CD method with improved real-time characteristics.
There is still a problem, however, in that real-time characteristics deteriorate with the conventional system as described above if the number of stations is increased.
In FA and LA systems, the data includes a portion for which real-time characteristics are required, and a portion for which such characteristics are unneccessary. Therefore, if attempt is made to transmit all the data without discrimination, it is not possible to ensure correct response for the portion of data for which real-time characteristics are required as the amount of data increases. This tendency is increasingly marked when there are many stations, and transmission cycles are therefore longer.